A Frogberry resembles a tomato.
Frogberries are a type of fruit only able to grow on the enchanted soil of the Forest of the Elves. They can be found growing among the ruins near Leaf's hot tub on the mentioned island, but they have wilted due to Leaf devoting all the water to his hot tub. Petal asks you to help give back water to the Frogberries by reusing Leaf's water pump's piping in order to get the necessary ingredients for a broth to cure Hopper from his insanity.
Buddy brings up that the Frogberries much resemble tomatoes. Technically in the game's programming, the Frogberries are actually the same object as the tomatoes on Chef Gino's farm. When mousing over the Frogberry/Tomato in the Wandolier catalog, it simply is referred to as "Fruit".